Researchers find a way to keep quantum memory and logic in synch

Scientist show how to make atoms swing to the beat (frequency).

A quantum computer, like any other computer, requires a way to store and retrieve information. In other words, some sort of memory. But because of the rich quantum entanglement gooey center of quantum computing, the memory and the logic need to be linked in a manner that's very different from that in classical computing: the magic of entanglement.

Physicists have been crowing about how they can create entangled states for a while now. Unfortunately, quantum computing requires something more: a memory state should last for a long time, independently of the logic parts, while the logic part should be accessible and fast. This makes coupling these two elements together in a useful way difficult... until now, that is.

Its the decoherence, stupid

Quantum computing, at a practical level, is all about how predictably things change with time, called coherence. If you give a swing a push, then, after a few measurements of the swing's position, you can use that data to predict the swing's position at any time in the future. This is because the eddies in the air motion around the swing simply don't have much effect on the swing's motion. In the quantum case, the qubit is in a superposition of two states, and the relative probability of measuring the qubit in one of them changes predictably with time (and, in fact, changes very much like the position of the swing with time).

Now imagine that some obnoxious shot-putter is throwing large metal balls at the swing after you give it a push. Every time one hits the swing, it noticeably changes the swing's motion and, in a very short time, you can't use previous measurements to predict the future.

The quantum world is filled with tiny swings that we can set into motion. Unfortunately, it is also filled with obnoxious, swing-hating shot-putters. We may set a qubit into motion, but the predictability of its behavior decays very, very quickly.

Fortunately, it doesn't have to be this way. What we can do is give the qubit an additional push every now and again to keep its motion predictable. We simply need to make sure that the pushes are always at the right time in the cycle (just like pushing a swing) and occur often enough that our pushes are more significant than the random lurches due to the shot-putter.

This technique allows one to extend the coherence of a qubit by quite a bit, and is well known in physics (your local MRI scanner will likely use the same technique).

Physicists have known how to extend the coherence of a qubit for a long time. But in quantum computing, you may want to store two entangled qubits that evolve at different rates. Then, extending the coherence of one is likely to destroy the coupling between the two qubits. In other words, if we preserve the coherence of our logic qubit, we cannot connect it to the memory qubit, and, if we don't extend the coherence, it will destroy itself before any memory operations can be completed.

All you coherence are belong to us

To solve this problem, researchers from a number of institutions took a more careful approach to the preservation of coherence. They realized that, even though the two qubits evolve at their own pace, there are periodic moments where they are at the same point in their cycle. If they give qubit a push at just that moment, then they still preserve the coherence of the logic qubit, but do not destroy its entanglement with the memory qubit.

You can think of this in terms of music. If you take two strings on a guitar and tune them so that they are nearly—but not quite—the same frequency, then you will not only hear both tones, but they will oscillate in volume. This oscillation is the frequency difference between the two vibrations, and creates what are called beat notes. Our quantum system has the same beat notes, and if we choose to apply a push to the qubit at the same point in this beat note oscillation, we will preserve the coherence of the fast decaying qubit (logic) without altering the state of the slow decaying qubit (memory).

In this case, the researchers were working with nitrogen vacancy centers in diamond—not guitars. Diamond consists of carbon atoms, arranged such that each carbon atom is connected to four others. But when nitrogen is introduced, one carbon atom is spurned by the nitrogen, leaving an electron that has a set of unique levels.

The spin—the direction the electron's internal magnetic field is pointing—provides a nice pair of states that can be set and read using light. Although it's great for computation, the electron is very sensitive to the environment and its coherence never lasts very long (typically a microsecond or so), so microwave pulses are used to flip the spin and preserve the coherence.

The nitrogen nuclear spin can also be used as a qubit. Because the nuclear magnetic moment is much smaller, it is much less sensitive to the environment, so it stays around for milliseconds—just what we want for memory. However, it takes a few microseconds to perform an operation on a nuclear spin qubit, by which time the electronic qubit has long since gone off to do other things.

Nevertheless, the electron's coherence lasts long enough that the nuclear spin undergoes many oscillations before it decays away. So, the researchers identified times in those oscillations when the microwave pulses would have no influence on the nuclear spin—and hit the electron with spin flips that coincide with those points. That is, they hit the beat note between the slow nuclear spin and the fast electron spin.

They demonstrated that they could perform basic logic operations, and performed one of the classic "look, we made a quantum computer" demonstrations: a search of a database containing four items. Although that sounds a bit cynical, this is a significant step, because it provides a convenient and natural link between a qubit that is near-perfect for logic operations with a qubit that is near-perfect for memory operations.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.